My system 80 (Black label, early model) Is getting some upgrades. First off I have fitted a socket for the fourth ROM position - Z13. To test this I hacked some code together which puts a few characters on screen, blew a 4K (2732) EPROM and popped it in. The machine is fine with no EPROM in the socket but crashes on power up with the EPROM fitted. The EPROM verifies and otherwise checks out electrically.

I read in a technical document that the socket is compatible with standard 27 series EPROMS, and human nature being what it is I have not bothered to question this.

Is there piece of information missing? Some systems require their utility ROMS to have a special header, for instance.

So after a second look it's apparent that the EPROM pinout for the '16 and '32 are roughly compatible, the active low select signal from pin 5 of Z22 joins pins 20 & 21, resulting in A11 being driven low along with the !CE pin. After fitting the missing pull-up R15 the machine boots with the EPROM in place woohoo!

The EPROM contains a tiny routine which preserves registers then fills the screen with the letter A. Registers are restored and a return executed.

I tried executing the code:

SYSTEM*? /12288

but the machine hung. PEEKing address 12288 gives 255. As does peeking the remaining 2048 addresses I removed the chip and once again verified it in a programmer. I also replaced the chip with a different brand - a Fujitsu this time - with the same symptoms.

I've looked at the chip select for the EPROM and it pulses as expected if I execute a BASIC loop to PEEK from it, so I think that part of the equation is sound. I'll stick the tickler on there this afternoon and see what it reveals.

can you get a 2716 , Program it with a known code, and test that to confirm that the machine is capable of working ?Ebay has them for about $1.75 NZ from china.

A late addition :- This is an example of what i had to do with an unknown 2716 Eprom i had, which a standard eprom Programmer/ reader could not read.Andrew Quinn suggested i use this method (Thanks Andrew).

ROM4S80_CIMIs a dump of the Z13 Rom as installed on my System-80 Educator Model.Under Newdos you would use the DUMP Command :-DUMP ROM4S80/CIM :0 3000H, 37FFH, FFFFHThis will give you a dump of the Rom Plus the entry address of the dump.at the beginning of the file.These 2 bytes have to be removed to dump it back to an Eprom.I use a Apple Mac and use a program called " HexEdit " which has the ability to do a destructive backspace to remove the 2 bytes.ROM4S80F.CIM Is the fixed version ready for Burning to a 2716 EpromThis has a CheckSum of E240h (on G540 Eprom programmer)Good luckRay

The EPROM socket Z13 requires some re-wiring on early model 1 motherboards. I've compared schematics from issue 1 (black label/no vu meter) to blue label issue boards and it's apparent where the problem lies. The bus buffers are not being enabled for read accesses to the address range served by the EPROM. It can be made to work though, so I'll persist at my leisure. Right now however I have other neat things to distract me..